Not to labour this point but I did read some articles on web regarding clustering which stated that each server had a C:\ drive where the software (ie SQL) was installed and that all the servers in the cluster must have the same software folder (e.g C:\Program Files\MSSQL7). This may be a difference between active/passive and active/active options, I don't know, I'm only a pleb where clusters are concerned.
I attached a url in my first post that seemed to dispute the statement that you cannot backup to C:, it stated that there could be a bug or feature of EM that does not show C: in it's list, but you can specify the full path insteads and it would work.
However since Brian has tried this and must know all about clustering, the info I found must be out of date or wrong.
Nice to know someone has done this as my boss keeps mentioning such things to me as if I am the fountain of all knowledge (which I ain't).
Oh! to be a guru and know all the answers eh!
Far away is close at hand in the images of elsewhere.